Self-Induced Fault of a Hydraulic Servo Valve - A Possible Cause for Hidden Malfunction of Aircraft's Systems

Acknowledgements

Contents

Abbreviations

Introduction

Topics and Arrangement of This Report

1 Occurrence and Suspicion

1.1 Occasion for Investigation, Brief Description of the Occurrence

1.2 Appraisal and Fundamental Contemplation for a Pressure-Controlling Servo Valve

1.2.1 Design Shape/Working Principle of the Hydraulic Valve

1.2.2 Characteristic Diagram—Static and Dynamic Demanding and the Signal Answer

1.2.3 Significant Influence Factors and Fault Sources of a Servo Valve

1.2.3.1 Particle Contamination of the Fluid (—A Widespread Superstition)

1.2.3.2 Robustness of the Torque Motor (First Stage)

1.2.4 Fault Potentials and Parameters in the Spool and Sleeve Assembly (Second Stage)

1.2.4.1 Dormant Fault Potentials Hidden in Design Shapes

Dimensional Tolerances at the Spool and Sleeve Assembly

Importance of the Balancing Grooves for Spool’s Working Ability

1.2.4.2 Significant Parametric Circumstances Whilst Operating

Changing of Actual Friction on the Sliding Surface Area Whilst Operating

Internal Leakage and Gap Tolerance as a Significant Influence Factor

Air/Temporary Vacuum in the Hydraulic Circuit

1.2.5 Fault Potentials/Possible Circumstances in a Solenoid Valve (First Stage)

1.3 Existence of a Self-induced Fault of a Two-Stage Servo Valve—Hypothesis

2 Survey and Supposition

2.1 Experimental Investigation—Test Execution and Results

2.1.1 Test Set-Up and Test Method

2.1.2 Measurement Data Analysis and Interpretations

2.1.2.1 Abnormality in the Pressure Response and Interrelations

2.1.2.2 Stability of the Pressure Signal Answer and Mobility of the Spool

2.1.2.3 Switching-Off Dynamic of the Spool

2.1.2.4 Feedback Intensity in the Water Hammer Effect

2.1.2.5 Fluid Dumping Behaviour at the Control Circuit

2.1.2.6 Hegemony Loss of the First Stage, Completion of the Self-induction

2.1.2.7 Development of a New Drainage and the Loss of Command Ability

2.2 Reconstruction of the Entire Fault Working Mechanism—Interim Conclusion

3 Supplements and Inference

3.1 Additional Reflections/Supplementation of Contemplation

3.1.1 Basic Working Mechanism of a Spool and Sleeve Assembly/Homologous Model

3.1.1.1 Mobility of a Spool/Equilibrium at a Working Point

3.1.1.2 Confinement of the Spool’s Freedom Grade/Guided Sliding of the Spool

3.1.1.3 Equilibrium and Meta-Equilibrium in the Second Stage

3.1.2 Origin of the Fault/Interlock Mechanism

3.1.2.1 Order of the Fault Induction/Origin of the Fault

3.1.2.2 Self-stabilization of the Flapper/Interlock of the Entire Servo Valve

3.1.2.3 Danger of the Flow Separation on the Flapper Body and the Timing

3.1.3 Initializing, Propagation and Escalation of the Blemish/Reason for Blemish

3.1.3.1 Degradation/Collapsing of the Lubrication Film—‘Dry Friction’

3.1.3.2 Holistic Consideration Incl. The Reaction of the Control Loop

3.1.4 Arbitrariness of the Fault—Russian Roulette Effect

3.2 Conclusion and Summary

3.2.1 Conclusion

3.2.2 Summary

Epilog

References